Cross arms are used throughout the world as structural elements to support electrical power transmission lines above the ground. These transmission cross arms, normally between 6 to 14 m in length, can be made of a variety of materials, the most common of which is treated wood.
The service life of cross arms is a very important factor. Given the difficulties of reaching and replacing the cross arms (which may be in very remote locations), the cost of replacing a cross arm often exceeds that of the cost of the cross arm, itself.
The use of timber cross arms poses certain challenges. Good quality timber for use in the cross arm is becoming increasing difficult to obtain given diminishing old growth forests, which is the prime timber source, as well as the impact of modern environmental laws.
Timber cross arms also have a limited life span (typically about 25 years) and decay naturally. The life span of timber cross arms may be enhanced by the use of wood preservatives, such as Creosote, Penta and CCA, however, these preservatives are not environmentally friendly, and may be toxic. In particular, many wood preservative treated wood products are banned for use in certain areas or industries.
Further, it is difficult to determine the state of a timber cross arm in service and assess the remaining life through visual inspection. Defect in timber due to insects and pests, moistness and/or temperature of the ambient surroundings of the timber, may be hidden and lead to costly asset failures and electrical system outages.
Timber cross arms are combustible and propagate fire rapidly in forest fires; they are attractive to woodpeckers; and, under certain weather conditions, such as lightening, for example, they can initiate a pole top fire leading to electrical system outages. Timber cross arms also creep (e.g. deflect) under heavy loads sustained for long periods of time.
There have been several attempts to overcome these difficulties by substituting timber with other materials. Despite these attempts, timber remains the primary source of cross arms in the power transmission industry.
Metal, particularly galvanized steel cross arms, have been used in order to overcome some of the disadvantages of timber. The primary disadvantage of using a metal cross arm is its electrical conductivity, which makes the cross arm very dangerous for transmission line technicians (or linemen) to work with on energized live lines. The galvanized coating of such cross arms has a life expectancy of about 25 years, after which the cross arm is susceptible to corrosion. In addition the commonly used steel sections are heavy and require significant lifting capacity in the field to install them. For these reasons, metal cross arms are not widely used.
Laminated timber has also been used for cross arms. Laminated timber is coated with a protective coating in order to generally prevent moisture penetration and increase the life expectancy of the cross arm. Some coatings are environmentally unfriendly, and may leach into the surrounding environment. Further, moisture and cracks may cause delamination of the timber. Under many circumstances, such cross arms may have a lower life expectancy than untreated timber.
Concrete, while commonly used as a building material, has not proven suitable for use as a cross arm. Concrete has large capillarity porosity, which allows water to penetrate and can cause the concrete to crack in freezing and thawing cycles. Unreinforced concrete will crack under tension stress. Regular concrete without reinforcement is quite brittle, and lacks ductility, which is a problem when used as a long cross arm. Given the different load conditions in electrical transmission lines (load due to the weight of conductors, insulators, radial ice on conductors, wind on conductors) the cross arm requires ductility, i.e. the ability of the material to plastically deform while continuing to carry loads without fracture, even after micro cracking. Also, concrete is not easily usable with thin sections of a cross arm. A cross arm made of concrete would be large, bulky, heavy and would require steel reinforcement for structural bending capacity and stirrups for shear reinforcement. For at least the above reasons concrete has not generally been used for cross arms across the transmission industry.